Creating a procedural stone texture
I have made a node setup that renders like this:



It consists of two diffuse shaders. One is your stone texture from above (well almost). The other is a simple green "dirt" material. The real trick is to combine the dirt with the stone in a believable pattern. I tried multiplying two noise textures with some distortion. Distortion makes noise be liked two colors of paint where you have stirred it some. You get a swirling pattern.

To get this further, I would add some bump to the dirt shader. Also, I would bake an AO map and use that in the mask, letting it influence the mask so that there is more dirt in crevices.

Here is the node setup.